There are many fields in which the absolute intensity of vacuum ultraviolet radiation must be measured. For example, in basic atomic and solid state physics research, the determination of the ability of radiation to ionize gases, and the measurement of absolute photoelectric emission yields of materials requires measurement of vacuum ultraviolet radiation. Of particular research importance is the determination of absolute absorption and photoionization cross-sections of various gases.
Prior to the development of parallel plate, high field strength ion chambers, measurement of radiant energy in the vacuum ultraviolet spectral region was difficult and generally unsatisfactory. Thus, although thermocouples which have been calibrated against a standard lamp traceable to the National Bureau of Standards are normally used to measure radiant energy in the visible and infrared spectral regions, and, although such thermocouples have been used to measure radiant energy in the vacuum ultraviolet spectral region, they are extremely insensitive and difficult to use at these wavelengths. Moreover, such thermocouples must be calibrated with visible light and an assumption made that the calibration remains valid for use of the thermocouples with vacuum ultraviolet radiation. As a consequence, measurements with thermocouples in the vacuum ultraviolet region have been either unobtainable, or inaccurate and unreliable.
In the X-ray spectral region, absolute radiation measurements have been made through the use of geiger counters and proportional counters. However, these devices cannot be used for absolute measurements in the vacuum ultraviolet region between 100 and 1020 A.
For wavelengths between 1020 A and 1400 A, ion chambers of various designs using "windows" of lithium or magnesium fluoride have been used as detectors but this type of ion chamber must be calibrated against a thermocouple.
With the development of parallel plate, high field strength ion chambers, measurements of the absolute intensity of vacuum ultraviolet radiation became possible without having to rely on other standards for calibration. Various embodiments of such ion chambers are described in Samson, "Absolute Intensity Measurements in the Vacuum Ultraviolet," 54 J. Opt. Soc. Am. 6 (January 1964) which is hereby incorporated by reference. When such devices are used as "double" ion chambers, their accuracy is extremely high, since fluctuations in the intensity of the radiation source do not affect the measurements. In addition, such double ion chambers provide the most precise method of measuring absolute absorption cross-sections in the spectral regions where the particle energies are greater than the ionization threshold of the gas being measured. When parallel plate ion chambers are being used as simple detectors, they have the further advantage of being wavelength selective. However, there are several features of the prior art parallel plate ion chambers which limit their usefulness and accuracy.
Whether a "single" ion chamber utilizing a single ion-collector electrode, or a "double" ion chamber utilizing a pair of ion-collector electrodes, parallel plate ion chambers are characterized by plate-like ion repeller and collector electrodes in substantially parallel arrangement with respect to each other. Assuming a substantially uniform charge distribution on the respective electrodes, the electrical field formed between the repeller and collector electrodes is characterized by a linearly increasing voltage potential, i.e., a uniform or constant potential gradient. Since high potential differences between repeller and collector electrodes are necessary to provide sufficient retarding voltages to prevent the collection by the ion collector electrodes of the relatively high energy electrons formed when gases are ionized by vacuum ultraviolet radiation, it is not possible to locate the photon beam from the radiation source between the repeller and collector electrodes so that the electrons generated by ionization of the gas do not acquire significant amounts of energy in travelling to the ion repeller electrode. As a consequence, the short-wavelength limit of parallel plate, high field strength double ion chambers; i.e., the wavelength at which secondary electron ionization of the gas occurs, is approximately 330 A. This figure is well above the possible limit, which is 250 A, thus limiting the usefulness of such devices.
It is an object of the present invention, in accordance with one aspect thereof, to provide ion chambers capable of measuring vacuum ultraviolet radiation which approaches 250 A in wavelength, which chambers may be of either single or double collector electrode configuration.
Prior art double ion chambers are further characterized by the inclusion of at least one guard ring electrode, and usually multiple guard ring electrodes, to ensure that each ion collector electrode will only collect ions formed vertically above it. In addition to increasing the complexity of ion chamber design and construction, the use of guard rings limits the minimum path length which can be provided for the ionizing photon beam to travel between the repeller and collector electrodes. This fact limits the utility of double ion chambers where the gas to be ionized is strongly photon absorbing. Further, since the presence of guard rings necessarily increases the length of the double ion chamber, the region in which the electrons produced by the ionization process can cause secondary ionization is also increased. Another disadvantage of guard rings is the fact that the collector electrodes must have minimum width and length dimensions in relation to the dimensions of the guard rings in order for the guard rings to be effective. As a consequence, the reliability of such ion chambers is reduced, since the greater the surface area of the collector electrodes, the greater the probability is that scattered photons from the photon beam will strike the collector electrodes and thereby generate erroneous ion current measurements. It is a further object of this invention, in keeping with another aspect thereof, to provide improved ion chambers which do away with such guard rings, and which in the case of double ion chambers, are of symmetrical design both in general construction and in electric field.